Transmitter for an optical communication signal

ABSTRACT

A transmitter for an optical RZ-DPSK communication signal comprises a source for an optical carrier, an electro-optical modulator which comprises at least one element having an optical path length adapted to be varied by an electrical driver signal for intensity modulating the optical carrier based on the driver signal, and a driver circuit for generating the driver signal from an electrical communication signal. The driver signal is an impulse-type signal having two types of impulses spaced in time by a neutral signal state, wherein in the presence of the neutral state of the driver signal at the modulator, the transmission of the modulator becomes zero, and the two types of impulses each cause a transmission different from zero and a phase which is specific for the type of the impulses in the modulator.

The present invention relates to a transmitter for an optical RZ-DPSKcommunication signal. Such transmitters are used for transmission ofcommunication signals at a high data rate on optical fibres. Opticaltransmitters for generating phase shift keyed signals generally comprisea laser for generating a narrow-band optical carrier and a modulatorwhich receives the optical carrier from the laser and keys phase shiftson it based on a communication signal applied to it, in whichinformation is encoded. The intensity of the carrier after its passagethrough the phase modulator is not modified by the modulation. Animprovement of transmission quality, i.e. an improvement of the errorrate and/or an increased range of the transmission with unchanged signalpower can be achieved by imposing an RZ envelope onto such a signal, sothat the symbols of the transmitted signal are separated from each otherby a time interval in which the intensity of the transmitted signalbecomes zero.

Prior art optical transmitters for a phase shift keyed RZ signalgenerally have the structure shown schematically in FIG. 1A. A laser 1acts as a source for a carrier wave of constant power which is ledthrough a phase-modulator 2 where phase shifts are keyed on it whichcorrespond to information bits of a binary, usually electriccommunication signal DATA supplied to the phase modulator. The phasemodulator 2 comprises a waveguide section from a bi-refringent materialsuch as lithium niobate, the index of refraction of which varies underthe influence of an electrode supplied with the electric communicationsignal DATA, and which can therefore assume two different levels ofoptical path length according to the level of the communication signalapplied to it. The output signal M of the phase modulator has a constantpower and is formed of a series of sections that may take two differentvalues of the phase shift with respect to the carrier wave provided bylaser 1, represented in the diagram of the signal M in FIG. 1A ashatched and unhatched sections, respectively. The different phase shiftscorrespond to diametrically opposed points in a constellation diagramshown in FIG. 1A. The transition between two sections of different phaseis not instantaneous, but requires a short time interval in which thephase of the output signal M of the modulator 2 changes continuously.I.e. in these times the state of the modulated signal M moves in theconstellation diagram on a unit circle on which the two phase statescorresponding to a symbol are located.

In order to suppress transition times of undetermined phase in signal M,the phase modulator 2 has an intensity modulator 3 connected to itsoutput, which is supplied with a clock signal CLK, the frequency ofwhich corresponds to the bit frequency of the communication signal data.The intensity modulator 3 provides a transmission signal T to be outputon a waveguide in the form of a series of impulses, which are separatedby time intervals with zero intensity and which can have two phasestates with phases shifted by 180°.

In another known embodiment of an optical transmitter for a RZ-DPSKcommunication signal, the phase modulator 2 is replaced by aninterferometer 4, in which at least one of its two arms has an opticalpath length that can be modified by the communication signal S. Theoutput signal of the interferometer obtained by superimposing thepartial signals transmitted on the two arms of the interferometer canassume different amplitudes according to the amount of the path lengthdifference between the two arms, but it has only two possible phasevalues at all times, including transition phases between two symbols.The output signal of the interferometer 4 therefore has no constantenvelope, but at each change of phase, the power of the modulated signalM goes through a minimum. In order to form an RZ signal from the outputsignal of the interferometer 4, it is conventional to lead the latterthrough an intensity modulator 3 which is supplied with the clock CLK ofthe communication signal DATA, just like the phase modulator 2 in FIG.1A.

The two known designs of a RZ-DPSK transmitter therefore require twooptical modulators, which are expensive and require a lot of space on acircuit board.

The object of the present invention is to provide a transmitter for anoptical RZ-DPSK communication signal, which is economic in manufactureand requires little space on a circuit board.

The object is achieved by a transmitter having the features of claim 1.

The electro-optical intensity modulator according to the presentinvention must comprise at least one element, the optical path length ofwhich is adapted to be modified by the driver signal, in order to beable to generate not only a change of intensity at constant phase of theoptical carrier modulated with the communication signal, but also azero-transition of the intensity accompanied by a phase reversal. Suchan electro-optical modulator may be formed in a manner known as such asan interferometer in which at least one arm has an optical path lengthadapted to be controlled by the driver signal, but it is alsoconceivable to use a Faraday rotator to which the driver signal isapplied, in combination with a subsequent linear polarizer.

In order to simplify the restoration, at a receiver side, of thecommunication signal from the output signal provided by the transmitter,the driver circuit of the transmitter advantageously comprises adifference circuit, which supplies a signal, referred to in thefollowing as a pre-coded signal, which is representative of thedifference between two subsequent bits of the electrical communicationsignal, and from which the driver signal applied to the intensitymodulator is derived.

This difference circuit may simply be formed by a XOR-gate and aflip-flop.

The signal processing circuit may simply and conveniently be formed withfour pairs of switches, each of which has a first and a second main portand a control port, wherein in each pair the first main ports of theswitches are connected to each other and the control ports of theswitches are supplied with mutually inverse input signals, wherein in afirst and a second pair the second main ports are connected to twooutputs of the driver circuit, and in a third and fourth pair, one ofthe two main ports is connected to one of the two outputs and the othersecond main port is connected to a first main port of the first andsecond pair, respectively. In such a switch array, the input signal ofthe first and second switch pairs may be a clock signal, and the inputsignal of the third and fourth switch pairs can be the pre-coded signal;conversely, the input signal of the first and second switch pairs may bethe pre-coded signal, and the input signal of the third and fourthswitch pairs may be a clock signal.

In order to achieve an optimum range of the communication signalgenerated by the transmitter and/or an optimum signal-noise-ratio at areceiver of this communication signal, it is desirable to have means forvarying the duty cycle of the communication signal, which enable tooptimise the duty cycle for given application. Such means may e.g. beformed by a mono-flop, the dwell time of which in the instable state iscontrollable.

Further features and advantages of the invention will become apparent byway of example from the subsequent description of embodiments referringto the appended drawings.

FIGS. 1A and 1B, already discussed, are block diagrams of conventionalRZ-DPSK transmitters and constellation diagrams thereof;

FIG. 2 is a block diagram of a transmitter according to the invention;

FIG. 3 shows schematically the structure of the interferometer of FIG.2;

FIG. 4 is an exemplary circuit diagram of the driver circuit of FIG. 2;

FIG. 5 is an alternative circuit diagram of the driver circuit of FIG.2; and

FIG. 6 is a block diagram of a receiver that is complementary to thetransmitter of FIG. 2.

The optical transmitter of the invention shown in FIG. 2 comprises adriver circuit 6, which generates from an arriving electrical two-levelcommunication signal DATA a pre-coded signal, the bits of whichcorrespond to the difference between two subsequent bits of thecommunication signal DATA. By inserting time intervals with zero-levelbetween the bits of the pre-coded signal D, a RZ-pre-coded signal isobtained, which is output from driver circuit 6 as a driver signal T tomodulation input of an interferometer 4. An optical input of theinterferometer 4 has a laser 1 connected to it which forms a source fora monochromatic optical carrier wave of constant power, on which thedriver signal T is to be modulated.

FIG. 3 schematically shows the structure of an interferometer 4 of theMach-Zehnder type. Two parallel waveguide branches 7, 8 connect theoptical input 9 to an optical output 10. Each branch 7, 8 contains aPockels cell 11, 12 in the form of an optical waveguide section made ofa material such as lithium niobate, the index of refraction of which isvariable for the polarization of the carrier wave supplied to opticalinput 9 under the influence of an electrical field that is generated bya voltage applied to electrodes 13. One of the two electrodes 13 of eachPockels cell is grounded, and the other is supplied with a DC voltageBIAS, which is selected so that the optical path length of the twobranches 7, 8 differ by λ/2, wherein λ, is the wavelength of the carrierprovided by laser 1, and it is connected in a DC-uncoupled way to one oftwo conductors 14 a, 14 b that form a symmetric input for the driversignal T. The amplitude of the impulses of the driver signal T isselected such that the impulses cause a change of the optical pathlength of λ/2 in the Pockels cell 11, 12 to which they are applied. Whenthe driver signal T has zero-level, the components of the carriertransmitted on the different branches 7, 8 interfere destructively atoutput 10, so that no optical power is transmitted. If an impulse of thedriver signal T is present, the two components interfere constructively,a transmission signal X is provided at output 10 which assumes oppositephases, depending on which one of the two conductors 14 a, 14 b has theimpulse applied to it.

An example for a structure of the driver circuit 6 is shown in FIG. 4.The communication signal DATA, which is initially assumed to beasymmetric, is applied to an input of a XOR-gate 17. Symmetric outputsof the XOR-gate 17 are connected to symmetric inputs D, D of a Dflip-flop 18. Clock inputs C, C are connected to a symmetric clocksignal CLK. The D flip-flop 18 has symmetric data outputs Q, Q, theinverting output of which is fed back to the second input of theXOR-gate 17. Thus, the XOR-gate forms the (unsigned) difference betweena present bit of the communication signal DATA and a bit which is storedin the D flip-flop 18 and is output at the output port Q thereof. Thebit value output at output port Q is therefore always zero during a bitperiod, if in the previous bit period the bit at output port Q and thebit of the communication signal DATA were different, and it is one, ifthey were equal.

Data and clock output ports Q, Q, C, C of the D flip-flop 18 areconnected to a network of four pairs of transistors T1 to T8. Theemitters of each pair are directly connected to each other. The bases ofthe transistors T1, T2 of the first pair are connected to the outputports Q, Q, respectively, of the D flip-flop 18, just like those oftransistors T3, T4 of the second pair. Similarly, the bases oftransistors T5, T7 and T8, T6 of the third and fourth pairs,respectively, are connected to the clock signal C and the inverted clocksignal C, respectively. The collectors of transistors T5, T6 areconnected to the first one of the conductors 14 that form the output ofthe driver circuit, and to ground via a resistor R1; similarly, thecollectors of transistors T8, T7 are connected to the second conductor14, and to ground via a resistor R2. The emitters of the fourth pair T6,T8 are connected to the collector of T2, those of the third pair T5, T7to the collector of T3. The emitters of the first and second pairs areconnected to a supply voltage via transistors T9, T10 that are openduring transmission operation, and resistors R9, R10, respectively.

The switch network can have four different input states, namely Q=C=0;Q=0, C=1; Q=1, C=0 and Q=C=1. In the first of these states, thetransistors T1, T9, T5, T3, T10 are open, and both conductors 14 a, 14 bare connected via these transistors and the resistors R9, R10 to thesupply voltage, so that they are at the same level, which corresponds toa symmetric output signal of zero. In the state Q=0, C=1, thetransistors T1, T9, T7, T3, T10 are open, so that the conductor 14 a isat the supply voltage. Simultaneously, the transistors T6, T5, T4 areblocking, so that the conductor 14 b is grounded by R2. In the stateQ=1, C=0, both conductors 14 a, 14 b are connected to the supply voltagevia transistors T8, T2, T9 and T4, T10, respectively, so that, again,the output signal is zero. In the state Q=C=1, the transistors T6, T2,T9 and T4, T10, respectively, are open, so that the conductor 14 b is atthe supply voltage, whereas T8, T1 and T3 are blocking, so that theconductor 14 a is grounded. As can be readily seen, the network oftransistors T1 to T10 always provides a zero-level when the clock signalis C=0; and if the clock signal is C=1, an impulse appears either onconductor 14 a or 14 b, according to the value of the data signal Q.Thus, the driver signal T is obtained. The interferometer 4 driven bythis driver signal thus provides the transmission signal X schematicallyshown in FIG. 2 in the form of an impulse train, the impulses of whichare separated by time intervals with a signal intensity of zero, and inwhich the phase of the carrier may take two different values,represented in the Figure by the impulses being hatched or not hatched.

According to an advanced embodiment, a mono-flop 19 may be inserted inthe clock signal lines Q, Q before or, as indicated in the Figure by adashed rectangle, behind the flip-flop 18, the dwell time of which inthe instable state is controllable and shorter than the period of theclock signal. By such a mono-flop acting symmetrically on the signals Q,Q, the duty cycle of the transmission signal, i.e. the ratio between theduration of the impulses and that of the period of the transmissionsignal may be controlled.

In order to guarantee voltage levels of the driver signal T that providethe required delay of λ/2 at the Pockels cells 11, 12, an amplifier 15may be inserted between the driver circuit 6 and the interferometer 4,as shown in FIG. 2.

FIG. 5 shows a second example of a driver circuit for a transmitteraccording to the invention. The components 17, 18, 19, T9, T10, R1, R2,R9, R10 are identical to those of FIG. 4 in arrangement and function,and are not described anew. Data and clock output ports Q, Q, C, C of Dflip-flop 18 are connected to a network of four pairs of transistors T1to T8. The emitters of each pair T1, T2; T3, T4; T5, T7 and T6, T8,respectively, are directly connected to each other. The bases of thetransistors T1, T2 of the first pair are connected to the clock signalC, C, respectively. Similarly, the bases of transistors T3, T4; T5, T7and T8, T6 of the second, third and fourth pairs, respectively, areconnected to the output Q, Q, respectively, of D flip-flop 18. Thecollectors of transistors T4, T5, T6 are connected to a first one ofconductors 14 that form the output of the driver circuit, and to groundvia a resistor R1; similarly, the collectors of transistors T3, T8, T7are connected to the second conductor 14, and to ground via a resistorR2. The emitters of the pair T2, T6, T8 are connected to the collectorof T2, those of pair T5, T7 to the collector of T1 and those of pair T3,T4 to the supply voltage via transistor T10 and resistor R10. Theemitters of the first pair T1, T2 are connected to the supply voltagevia transistor T9 and resistor R9. In transmission operation, bothtransistors T9, T10 are open.

In the first of the four input states Q=C=0; Q=0, C=1; Q=1, C=0 andQ=C=1 of the switch network, the transistors T1, T3, T5, T8 are open,and the two conductors 14 a, 14 b are connected via these transistorsT9, T10, R9 and R10 to the supply voltage, so that a zero output signalis generated. In the state Q=0, C=1, the transistors T2, T3, T5, T8 areopen, so that the conductor 14 a is at the supply voltage.Simultaneously, the transistors T1, T4, T6, T7 are blocking, so that theconductor 14 b is grounded by R2. In the state Q=1, C=0, T1, T4, T6, T7are open, so that both conductors 14 a, 14 b are at the supply voltageand again, a zero output signal is generated. In the state Q=C=1, thetransistors T2, T4, T6, T7 are open, so that the conductor 14 b is atthe supply voltage, and the conductor 14 a is grounded. The behaviour ofthe driver circuit of FIG. 5 does not differ from that of the circuit ofFIG. 4.

In a receiver, as shown schematically in FIG. 6, the communicationsignal DATA is restored from the transmitter signal X. To this end, thelatter is distributed onto two fibres 21 a, 21 b at a directionalcoupler 20, and the signal in fibre 21 b is delayed with respect to thatof fibre 21 a by one bit period. Depending on whether the phases of twosubsequent impulses of the received signal X are equal or opposite,constructive or destructive interference occurs at a photo detector 23,downstream of the second directional coupler 22. The photo detector 23provides a pulsed output signal, the levels of which are equal to thoseof the signal DATA.

1. A transmitter for an optical RZ-DPSK communication signal,comprising: a source for an optical carrier; an electro-opticalmodulator having at least one element with an optical path lengthmodified by an electrical driver signal for intensity modulating theoptical carrier based on the driver signal; and a driver circuit forgenerating the driver signal from an electrical communication signal,wherein the driver signal is an impulse-type signal having impulses oftwo types spaced in time by a neutral signal state, wherein the impulsesof the two types have opposite signs, wherein during the neutral signalstate of the driver signal, a transmission of the modulator becomeszero, and the two types of impulses cause the transmission of themodulator to be different from zero and a phase shift which is specificfor each type of the impulses, wherein the driver circuit comprises fourpairs of switches, each having first and second main ports and a controlport, and wherein in each pair, the first main ports of the switches areconnected to each other and the control ports of the switches aresupplied with mutually inverse input signals.
 2. The transmitter ofclaim 1, wherein the specific phase shifts differ by π.
 3. Thetransmitter of claim 1, wherein the modulator is an interferometerhaving arms, wherein the optical path length of at least one of the armsis controllable by the driver signal, and wherein a neutral signal levelcorresponds to a path length difference between the arms of half of acarrier wavelength of the optical carrier.
 4. The transmitter accordingto claim 3, wherein two conductors are used for transmitting the driversignal, wherein the impulses of a first type are transmitted on a firstof the conductors, and wherein the impulses of a second type aretransmitted on a second of the conductors.
 5. The transmitter of claim4, wherein the two arms each comprise the element having a controllableoptical path length, the first of which is connected to the first of theconductors and the other of which is connected to the second of theconductors.
 6. The transmitter according to claim 1, wherein the drivercircuit comprises a difference circuit for forming a pre-coded signalrepresentative of a difference between subsequent bits of the electricalcommunication signal, and wherein the driver signal is derived from thepre-coded signal.
 7. The transmitter of claim 6, wherein the differencecircuit comprises an XOR-gate and a flip-flop.
 8. The transmitter ofclaim 1, wherein, in a first and second pair, the second main ports areconnected to two output ports of the driver circuit, and, wherein, in athird and fourth pair, one of the second main ports is connected withone of the two output ports, and the other second main port is connectedto the first main ports of the first and second pairs, respectively. 9.The transmitter of claim 8, wherein the input signal of the first andsecond switch pairs is a clock signal, and wherein the input signal ofthe third and fourth switch pairs is a pre-coded signal formed by adifference circuit of the driver circuit.
 10. The transmitter of claim1, wherein, in a first, second and third one of the pairs, the secondmain port is connected to two output ports of the driver circuit, and ina fourth one of the pairs, the common first main port is connected to asupply voltage, and each of the second main ports is connected to one ofthe common first main ports of the first and second pairs, respectively.11. The transmitter of claim 10, wherein the input signal of the first,second and third switch pairs is a pre-coded signal formed by adifference circuit of the driver circuit, and wherein the input signalof the fourth switch pair is a clock signal.
 12. The transmitteraccording to claim 1, and a controller for varying a ratio between aduration of the impulses and a duration of the neutral signal state. 13.The transmitter of claim 12, wherein the controller is a mono-floplocated in a clock line of the driver circuit.